Prevention of coke formation in pyrolysis of acetone



Jan 9 1951 L. A. NlcoLAl ETAL PREVENTION OF COKE FORMATION 1N PYRoLYsIsoF AcEToNE med June 4, 194e www5@ John. O. 5mdk b I @thor-nes PatentedJan. 9, 195! PREVENTION OF COKE FORMATION IN PYROLYSIS OF ACETONE LloydA. Nicolai and Edward W. S. Nicholson,

. Baton Rouge, La., and John 0. Smith, Coventry,

England, assignors to Standard Oil Development Company, a corporation ofDelaware Application June 4, 194s, serial No. 31,126

(ci. 26o-585.5)

6 Claims.

'Ihis invention is directed to a process for treating apparatus forcarrying out reactions, such as are used in the production of gases andvapors by the distillation and cracking of materials containing them, inthe vapor or liquid phase in metal wall reaction chambers or in thepresence of metal parts or elements. More particularly, this inventionrelates to the conditioning of a'lloy steels containing high percentagesof chromium and nickel which are to be used in the distillation and'cracking of materials in the vapor or liquid phase.

In commercial processes in which organic compounds are distilled orcracked in tubes, vats, chambers or containers composed of ferrousalloys containing high percentages of chromium and nickel, it is awell-known fact that the high nickel content of the reaction chambersurfaces results in the formation of excessive amounts of coke whichsoon fills the tubes, vats, chambers or the like, necessitating costlyshutdowns for purposes of cleaning out the coke.

It is, therefore, the "main object of this4 invention of provide amethod for conditioning the surfaces of a high chrome-high nickel steelprior to its use in the cracking of organic compounds so as tosubstantially reduce or even to eliminate coke formation.

According to the present invention the above difllcultles due to cokingare overcome by subjecting the inside surfaces of the metal tubes to anovel conditioning treatment which involves a starting-up procedureusing controlled conditions of repeated cracking and regeneration. Iheinvention contemplates the practice of reaction processes of thecharacter referred to, by preliminarily giving this treatment to themetal surface or to the interior surface of the metal piping, vat,chamber or container which is to be employed in the reaction process soas to condition the metal surface and thereby prevent excessive cokingwhen the metal surface is subsequently used in the above-describedprocess.

The invention will be described in connection with'the process ofcrackingketenizable material such as acetone, acetic acid, aceticanhydride or the like, to produce ketene, but this is intended toillustrate the method and the invention is not intended to be limited tosuch use. In general, all the prior art processes either definitelystate, or by setting forth examples indicate that the pyrolysis ofacetoneor other ketenirable material should be carried outin'carbon,copper, quartz, or silver pyrolysis'apparatus. While copperissatisfactory because 4of italien-catalytic activity with 2 respect toketene, from the structural standpoint and for high temperature use,copper presents certain disadvantages. For example, it may becomenecessary to sheathe or otherwise protect the copper reaction chamber inorder to prolong its life. Furthermore, copper oxidizes during periodsof regeneration and is not structurally strong. A high-chrome-no-nickelsteel fails upon repeated heating and cooling, unless tempered by slowlyheating or cooling each time. This is im practical in an emergencyshutdown, for example, when the temperatures have to be dropped rapidly.Carbon steel is unsatisfactory because it causes severe coking and isnot capable of withstanding temperatures over 1100 F. Ahighchrome-low-nickel steel such as an 18-8 chromenickel alloy isunsuitable for the same reason although it can take a slightly highertemperature.

Consequently it has been found desirable to use ahigh-chrome-high-nickel steel in the'reaction chambers when pyrolyzing aketenizable material to ketene. For example, an alloy of the followingcomposition has been found to be particularly suitable:

232'% chromium 12-20% nickel Less than 0.2% carbon Balance substantiallyiron Such a material has the advantage over copper in that it is notoxidized at operating temperature and it has the advantage over silverin co's'ts. Yields may be obtained that are as good as any obtained byusing copper or silver.

For better appreciation of the invention, reference is made to theaccompanying drawing in which numeral 2 represents a, vaporizer meansfor acetone, which may be a simple coil as shown, or a metal stillpot orthe like. The vaporizer is connected by line 3 to preheater 4 whichlikewise may be of simple construction, comprising merely a S-coil orthe like.

The preheater 4 is illustrated as a coil of large diameter to providelow pressure drop and may be composed of the alloy described above.However, it may, if desired, consist of rows of straight tubes connectedin series or parallel by means of return bends or headers. It isconnected by conduit 5 to pyrolysis chamber 6 which may be a single tubeor coil or other type of enclosed chamber, but is of less diameter thanthe presorption tower 8 which may be of any convenient construction.

Tower 8 ls provided with conduit means 8 for introducing acetic acid tobe reacted with the ketene and with draw-off means III at the base ofthe column connected with fractionator II. The upper part of theabsorber is provided with vapor take-off means I2 connecting withcondenser I3. Fractionator II is provided with bottom draw-off means I4and vapor take-off conduit I5 which passes through condenser I6 to thetop of absorber 8. Part of the condensate from condenser I6 is returned.to fractionator II through line I5A for reflux and at least a part ispreferably passed to absorber 8 through line 25. Condenser I3 isconnected with line I1 to scrubber I8 which is provided at its top withmeans I8 for introducing scrubbing liquid and vent line 20 for removinggasee and connects at its bottom with line 9 into the absorber 8. Theliquid condensed by condenser I3 is recycled by line 23 back to the feedinlet to the vaporizer and preheater mnes. Means are provided inabsorber 8 and fractionator II for supplying heat, such as steam coils2| and 22.

In operating the process of this invention foi. the continuous crackingof acetone, a positive gage pressure of about ve pounds is maintained onthe apparatus but with an inlet temperature of the gas to the crackingzone of about 1150 t0 1200 F. and an outlet temperature for the vaporsof about 1300 F.l400 F. when acetone is being cracked. The contact timeof vapors with cracking temperatures should preferably be between 0.25and 0.75 second, and the vapors preferably should have a mass velocityin the cracking zone of about 10-40 lbs/sq. ft./sec.

If the above appartus is immediately used for the cracking of acetoneunder the above conditions, the tubes of pyrolysis chamber 6 soon becomeplugged with coke. If, however, the tubes of chamber 6 are. firstsubmitted to closely controlled conditions of acetone cracking andtreatment with steam and air, the tubes become conditioned and no longerplug up with coke. In order to accomplish this conditioning acetone isintroduced into coil 6 at 10-50 lbs/sq. ft./sec. mass velocity, with acontact time of 0.1 to 1.0 sec. and temperatures between 1150 and 1400F. It is essential that the flow rate and pressures be held constantduring the cracking operation and under some conditions it is desirablethat the temperature be increased slowly from 1100o F. up to the naltemperature used. The rate of rise should be about 10 to 50 F. per hour.As soon as coking sets in, as indicated by increased pressure across thereaction tube, the acetone feed is cut oil?, the coil is flushed with aninert gas and subjected to treatment with a mixture of steam and air attemperatures between 1600o and 1800 F. until the coke is removed(regeneration). The treatment with acetone and then with steam and airis repeated under the same conditions after cooling, at least once andpreferably several times with a maximum total of about six after whichthe temperature in the regeneration step is reduced to 14001500 F. andthe same sequence of steps, under these reduced temperature conditions,are repeated until cracking occurs without pressure increase across thecracking coil. The above sequence of steps can be carried out with thetemperature in the regeneration step maintained throughout between 1400and 1500* F. The conditioning may be hastened by using a fasttemperature start-up during the cracking step, that is, the temperatureis taken from below the cracking range of about 4 1l00 F. to the finalcracking temperature in from fteen minutes to one hour.

In actual pilot plant operation, a new coil of the above composition wasconditioned by the following operations:

l. Regeneration with steam and air at 1550 F.

2. Cracking of acetone at 1200-1300 F. followed by regeneration at 1750F. with steam and air. This was repeated three times more, with coolingto 1100 F. prior to each cracking step.

3. Cracking of acetone at 1200-1300 F. followed by regeneration at 1450F. with steam and air. This was repeated once.

After the above steps, cracking of acetone was started with nodeposition of carbon in the tube and with very low dehydrogenationindicated.

While it is not intended to limit this invention by any theory as towhat occurs by the above described treatment, metallurgical examinationof the conditioned tubes indicated that a thi'n skin around theinner'surface of the tube had been changed by the addition of extraneouscarbon not present in the original metal and that carbon originallypresent in the metal had been precipitated on the grain boundaries.X-ray diffraction patterns did not show conclusively what the nature ofthe thin inner protective surface was, but it is believed that carbideformation had occurred.

A tube conditioned in the above manner should not be cleaned bymechanical means since it has been found that if the conditioned surfaceis scratched it is necessary to start the conditioning process again inorder to prevent coking. It is, therefore, preferable to burn out anydeposits in the tube rather than scrape or bore them out.

'Ihe above-described conditioning process while described in detail asapplied to the cracking of acetone to produce ketene, is equallyapplicable to other pyrolytic processes, such as the dehydrogenation ofbutane, in which high-chrome-highnickel steels are required forstructural strength at the high temperatures necessary for the reaction,but in which the nickel or chromium may catalyze undesirable sidereactions.

The nature and obects of the present invention having thus been fullydescribed and illustrated, what is claimed as new and useful and isdesired to be secured by Letters Patent is:

l. In a process for the manufacture of ketene by the pyrolysis ofacetone wherein a chemical reaction in a ferrous metal container, theinside surface of which is composed of zii-27% chromium, l2-20% nickel,and the balance substantially iron, results in excessive coke formationof the inner surface of the container, the method of conditioning saidsurfaces and preventing said excessive coke formation which comprisespreliminarily contacting said surface with acetone at a temperaturebetween 1150 and 1400u F. until excessive amounts of coke are formed onthe surface, subjecting the coke-covered surface of a regenerationtreatment with steam and air at a temperature between 1600 and 1700" F.until the coke is removed, repeating the pyrolyzing and regeneratingsteps several times under the same conditions and then repeating thesame steps for several more times while maintaining a temperature of1400-1500" F. in the regeneration step until coke no longer forms on thesurface.

2. In a process for the manufacture of ketene by the pyrolysis ofacetone wherein a chemical reaction in a ferrous metal container, theinside surface of which is composed of 23-27% chromium, 12-20% nickel,and the balance substantially iron, results in excessive coke formationof the inner surface of the container, the method of conditioning saidsurfaces and preventing said excessive coke formation which comprisespreliminarily contacting said surface with acetone, while increasing thetemperature from 1100 F. up to a temperature not lower than 1150 F. andnot over 1400 F. at the rate of 10 to 50 F. per hour, maintaining thenal temperature reached while continuing to treat with acetone untilexcessive amounts of coke are formed on the surface, subjecting thecoke-covered surface to a regeneration treatment with steam and air at atemperature between 1600 and 1700 F. until the coke is removed, coolingthe coke-free surface to 1100 F., repeating the pyrolyzing andregeneration steps several times under the same conditions and thenrepeating the same steps for several more times While maintaining atemperature of l4 00 to 1500 F. in the regeneration step until coke nolonger forms on the surface.

3. In a process for the manufacture of ketene by the pyrolysis ofacetone wherein a chemical reaction in a ferrous metal container, theinside surface of which is composed of Z13- 27% chromium, 12-20% nickel,and the balance substantially iron, results in excessive coke formationof the inner surface of the container, the method of conditioning saidsurfaces and preventing said excessive coke formation which comprisespreliminarily contacting said surface with acetone while rapidlyincreasing the temperature from 1100 F. up to a temperature not lowerthan 1150 F. and not over 1400 F. within 15 minutes to one hour,maintaining the nal temperature reached, while continuing to treat withacetone, until excessive amounts of coke are formed on the surface,subjecting the coke-covered surface to a regeneration treatment withsteam and air at a temperature between 1600 and 1700 F. until the cokeis removed, cooling the coke-free surface to 1100 F., repeating thepyrolyzing and regeneration steps several times under the sameconditions and then repeating the same steps for several more timeswhile maintaining a temperature of 1400-1500" F. in the regenerationstep until coke no longer forms on the surface.

4. Process according to claim 1 in which each and every regenerationstep is carried out at 1400-1500 F. throughout the conditioningtreatment.

5. Process according to claim 2 in which each and every regenerationstep is carried out at 14001500 F. throughout the conditioningtreatment.

6. Process according to claim 3 in which each and every regenerationstep is carried out at 14001500 F. throughout the conditioningtreatment.

LLOYD A. NICOLAI. EDWARD W. S. NICHOLSON. JOHN O. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,646,349 Curme Oct. 18, 19271,703,949 Norwood Mar. 5, 1929 1,941,271 Pollock .t. Dec. 26, 19331,962,502 Grebe et al June 12, 1934 2,215,950 Young Sept. 24, 19402,218,066 Boese Oct. 15, 1940 2,393,778 Hull Jan. 29, 1946

1. IN A PROCESS FOR THE MANUFACTURE OF KETENE BY THE PYROLYSIS OFACETONE WHEREIN A CHEMICAL REACTION IN A FERROUS METAL CONTAINER, THEINSIDE SURFACE OF WHICH IS COMPOSED OF 23-27% CHROMIUM, 12-20% NICKEL,AND THE BALANCE SUBSTANTIALLY IRON, RESULTS IN EXCESSIVE COKE FORMATIONOF THE INNER SURFACE OF THE CONTAINER, THE METHOD OF CONDITIONING SAIDSURFACES AND PREVENTING SAID EXCESSIVE COKE FORMATION WHICH COMPRISESPRELIMINARILY CONTACTING SAID SURFACE WITH ACETONE AT A TEMPERATUREBETWEEN 1150* AND 1400* F. UNTIL EXCESSIVE AMOUNTS OF COKE ARE FORMED ONTHE SURFACE, SUBJECTING COKE-COVERED SURFACE OF A REGENERATION TREATMENTWITH STEAM AND AIR AT A TEMPERATURE BETWEEN 1600 AND 1700* F. UNTIL THECOKE IS REMOVED, REPEATING THE PYROLYZING AND REGENERATING STEPS SEVERALTIMES UNDER THE SAME CONDITIONS AND THEN REPEATING THE SAME STEPS FORSEVERAL MORE TIMES WHILE MAINTAINING A TEMPERATURE OF 1400-1500* F. INTHE REGENERATION STEP UNTIL COKE NO LONGER FORMS ON THE SURFACE.